(1) Field of the Invention
The present invention relates to a novel method for crosslinking an isoprene-isobutylene rubber which is a synthetic rubber, as well as to a crosslinked rubber product obtained by the method.
(2) Description of the Prior Art
Isoprene-isobutylene rubber (hereinafter abbreviated to xe2x80x9cbutyl rubberxe2x80x9d in some cases) is produced by copolymerization of isoprene and isobutylene, has an unsaturation degree of 0.5 to 3 mole %, and is a known synthetic rubber. Crosslinked butyl rubber has low gas permeability, electrical insulation, heat resistance, damping property, resistance to acids and alkalis, low water absorption, etc., and is in use in rubber vibration insulator, automotive tube, packing, rubber stopper, o-ring, etc.
For crosslinking of butyl rubber, there have been known three methods, i.e. sulfur crosslinking, quinoid crosslinking and resin crosslinking. Of these, resin crosslinking is most preferred in order to obtain a crosslinked butyl rubber which satisfies the requirements for heat resistance, low compression set, high hardness, high electrical insulation, low corrosivity to metals, etc.
Regarding the technique for crosslinking of butyl rubber, there is the following description in p. 117 of xe2x80x9cPOLYSAR Butyl Handbookxe2x80x9d issued by Polysar Co. in 1996.
xe2x80x9cPOLYSAR Butyl compounds with exceptional heat resistance and low compression set can be obtained by curing with dimethylol phenol resins. Halogen-bearing activators are customarily used in conjunction with the resin, but it is possible to produce satisfactory cures without activation by curing at high temperature, especially with high unsaturation POLYSAR Butyl rubbers.xe2x80x9d
Thus, a butyl rubber becomes a crosslinked butyl rubber of good properties by adding only a dimethylphenol resin thereto and conducting crosslinking at 180 to 210xc2x0 C. The crosslinked butyl rubber has good properties in heat resistance, low compression set, electrical insulation and corrosivity to metals. However, when there is required a crosslinked butyl rubber having good electrical insulation and yet a high hardness, it is difficult to find an appropriate combination of raw materials enabling the production of such a crosslinked butyl rubber. That is, carbon black, which is used as a filler to impart a certain hardness to a crosslinked butyl rubber obtained, has an upper limit in the addition amount when the crosslinked butyl rubber must have electrical insulation; in such a case, therefore, use of carbon black alone is unable to allow the obtained crosslinked butyl rubber to have a desired hardness. Examples of the crosslinked butyl rubber product having a high hardness are a hard butyl rubber roller, a packing for high-pressure-water pipe and a sealing rubber for electrolytic capacitor.
In producing such a product, a silane coupling agent has been used, in addition to carbon black, to obtain a required hardness. The silane coupling agent includes silanes such as vinylsilane, mercaptosilane, aminosilane and the like. When a silane is added, the silanol group of the silane reacts with silica or clay (which is added as other filler), whereby a high strength and a high hardness are obtained; however, the hardness obtained is unstable.
For example, the hardness obtained differs depending upon whether a silane is added in a mixture with silica or clay or a silane is added separately. Further, when a closed type kneader (e.g. a Banbury mixer) is used and when a silane is added to a compound (a raw material mixture) of 150xc2x0 C. or higher, no stable hardness is obtained because the silane is vaporized or its reaction with silica or clay is unstable (thus, there are indefinite parameters). The reason therefor is thought to be that the reaction of the silane with silica or clay is difficult to control.
Production of a crosslinked butyl rubber of high hardness by resin crosslinking is possible by using a halogen compound (e.g. tin chloride or chloroprene rubber) in combination with a phenolic resin, when there is no upper limit to the amount of carbon black used. Use of a halogen compound, however, is not preferred depending upon the application of the crosslinked butyl rubber obtained, because the halogen compound may cause metal corrosion. Further, increase in carbon black amount for production of crosslinked butyl rubber of high hardness is not preferred, either, because the compound (raw material mixture) used for production of such a crosslinked butyl rubber has low moldability or because the crosslinked butyl rubber obtained has low electrical insulation.
Herein, xe2x80x9cbutyl rubber of high hardnessxe2x80x9d refers to a butyl rubber having a JIS-A hardness or durometer-A hardness of 80 or more.
The present invention aims at alleviating the above-mentioned problems of the prior art by providing (a) a novel method for crosslinking a butyl rubber with a resin without using any halogen compound and (b) a crosslinked rubber product of high hardness obtained by the method (a).
The first object of the present invention is to provide a novel method for crosslinking a butyl rubber with an alkylphenol-formaldehyde resin without using any halogen compound. The second object of the present invention is to provide a crosslinked rubber product of high hardness obtained by the above crosslinking method.
The third object of the present invention is to provide a novel method for crosslinking a butyl rubber with an alkylphenol-formaldehyde resin, wherein a hydrazide compound or a hydrazide compound and an epoxy compound are used in combination with the alkylphenol-formaldehyde resin and no halogen compound is used. The fourth object of the present invention is to provide a crosslinked rubber product which has good moldability, causes no metal corrosion, and is excellent in electrical insulation and high in hardness.
Other objects of the present invention will become apparent from the following description.
The above objects of the present invention are achieved by:
(1) a method for crosslinking an isoprene-isobutylene rubber, which comprises adding, to an isoprene-isobutylene rubber, an alkylphenol-formaldehyde resin and a hydrazide compound;
(2) a method for crosslinking an isoprene-isobutylene rubber, which comprises adding, to an isoprene-isobutylene rubber, an alkylphenol-formaldehyde resin, a hydrazide compound and an epoxy compound; and
(3) a crosslinked rubber product obtained by the above method (1) or (2).
In the above method (1) and/or (2), the amount of the isoprene-isobutylene rubber is 100 parts by weight; the amount of the alkylphenol-formaldehyde resin is 5 to 25 parts by weight; the amount of the hydrazide compound is 0.1 to 5 parts by weight; and the amount of the epoxy compound is 0.3 to 10 parts by weight.
The alkylphenol-formaldehyde resin is a compound represented by the following formula (1): 
wherein n is 0 to 10, R is an aliphatic alkyl group having 1 to 10 carbon atoms, and Rxe2x80x2 is xe2x80x94CH2xe2x80x94 or xe2x80x94CH2OCH2xe2x80x94.
The hydrazide compound is at least one kind of hydrazide compound selected from the group consisting of the dibasic acid dihydrazides and carbodihydrazide represented by the following formulas (2) to (5): 
[wherein X and Y may be the same or different and are each a hydrogen atom or an alkyl group having 1 to 5 carbon atoms; R is a hydrogen atom or a group represented by 
(wherein Rxe2x80x2 is a hydrogen atom, a methyl group or a hydroxyl group); n is a number of 0 to 2; and m is a number of 0 to 20 (n and m are not 0 simultaneously)], 
[wherein R is a hydrogen atom or an alkyl group having 1 to 10 carbon atoms; Rxe2x80x2 is a hydrogen atom or a group represented by 
(wherein Rxe2x80x3 is a hydrogen atom, a methyl group or a hydroxyl group); and n is a number of 1 to 10], 
[wherein R is a hydrogen atom or a group represented by 
(wherein Rxe2x80x2 is a hydrogen atom, a methyl group or a hydroxyl group)], and 
Specific examples of the hydrazide compound are carbodihydrazide, adipic acid dihydrazide, sebacic acid hydrazide, dodecanedioic acid dihydrazide, isophthalic acid hydrazide, maleic acid hydrazide, decamethylenedicarboxylic acid disalicyloylhydrazide, eicosanedioic acid dihydrazide, 7,11-octadecadiene-1,18- dicarbohydrazide, and 1,3-bis(hydrazinocarboethyl)-5-isopropylhydantoin.
The epoxy compound is at least one kind of epoxy compound selected from the group consisting of a bisphenol A type epoxy resin, a bisphenol F type epoxy resin, a bisphenol AD type epoxy resin, a bisphenol type epoxy resin obtained by substituting one methyl group added to the skeleton of bisphenol AD type epoxy resin, with an alkyl group having 2 to 12 carbon atoms, a phenolic novolac type epoxy resin, a cresol novolac type epoxy resin, a triphenylmethane type polyfunctional epoxy resin, an alicyclic epoxy compound, a naphthol-modified novolac type epoxy resin and an epoxy compound of glycidyl o-, m- or p-phthalate or o-, m- or p-hydrophthalate type.